I've had dozens od old chevys, fords and even dodges, the only problem with the tank behind the seat was the leaking old neoprene sleeve that needs to be replaced so you don't smell gas. The 80's had chevys with exploding saddle tanks, yup the exploding pintos and mustangs. No matter where you put the thing if hit you could go up in a flaming mushroom of trouble! To put it in perspective there was a bit on I think it was myth busters regarding different fuels people were experimenting with. They included bio diesel, hydrogen etc. The narrator mentioned something about having a tank of hydrogen under his *** in the vehicle and the professor stated what the heck do you think a 20 gallon tank of gas will do! I moved mine just to get some leg room, big pain for a few inches. By the way I used to smoke in all those old trucks never heard of an issue if you make sure that sleeve is in good shape. Good luck!
-Doug

 

Reading this got me thinking. It looks like in or under the cab might be the best place. Think about it. People have been mentioning smelling gas and not wanting that in the cab. If the tank is between the frame rails, you will be less likely to smell gas. So less likely to know to fix a serious problem. With your exhaust running all along the vehicle, you have a heat source everywhere.

And besides, I watched the movie Hot Shots!, (Or was it Hot Shots! part Deux) so I know what happens when a car with a tank towards the rear gets hit.

 

This was on the HAMB about 8 yrs ago. As I recall, he was going 65 +/- on a Cal. freeway, took a hit and it rolled a couple times after hitting the armco. Look at which area of the truck is intact...

 

the pinto deal was due to the Dana style rear end rupturing the tank. i think "the fix" was adding a bolt on piece of teflon to the top of the housing/rear cover so the tank didn't get punctured. if ford had put the 8" under the pinto like the mustang II mostly had, it prolly wouldn't have ever happened. the 8" had a rounded back and would be less likely to dig in.

i believe ford proved that if you rearend about any car through that era with a BIGGER vehicle at a high rate of speed you could get the same result. i'm not scared of a pinto or a behind the seat tank. i guess i'm a rebel....

 

The Pinto thing was that the tank straps were able to disengage from the frame and let the tank fall on the ground.